The silicon required for the production of solar cells must have a high level of purity. Due to the strong global increase in demand for high-purity silicon, the solar industry increasingly uses processed secondary silicon or metallurgical silicon and purified metallurgical silicon for the production of solar cells. Secondary silica are by-products of the semiconductor industry whose levels of purity are no longer sufficient for the semiconductor industry but meet the requirements for the production of solar cells after a corresponding treatment. The secondary silicon is in particular a silicon which contains high concentrations of doping elements such as boron, gallium, phosphorous, arsenic or antimony. The concentration of these doping elements needs to be reduced in the processing of the silicon.
Various prior-art methods are known for processing silicon containing doping elements, for instance slag extraction, crystallization of the silicon melt and low-pressure evaporation. The drawback of the mentioned methods is that several processing steps are necessary in order to achieve the desired purity of the silicon, thus resulting in correspondingly long processing times. Moreover, the mentioned methods produce large amounts of spontaneously igniting or toxic by-products which are difficult and expensive to dispose of. Finally, there are considerable losses of silicon.
A continuous method of producing purified silicon is disclosed in EP 0 530 567 A1 where the silicon to be purified is treated with reactive gases or reactive gas mixtures. Reducing the concentration of boron in the silicon to be purified is performed by means of the reactive gas hydrogen or by means of a reactive gas mixture which is composed of hydrogen and water vapor. The drawbacks of this method are that only the concentration of boron can be reduced to a sufficient degree, and that there are considerable losses of silicon as well.